1. Field of the Invention
This invention relates to an optical element, a lithographic apparatus comprising such optical element and to a device manufacturing method.
2. Brief Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, e.g. of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include, for instance, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
Some extreme ultraviolet sources, especially plasma sources, emit radiation over a wide range of frequencies, even including infrared (IR), visible, ultraviolet (UV) and deep ultraviolet. These unwanted frequencies will propagate and cause heating problems in the illumination and projection systems and cause unwanted exposure of the resist if not blocked. Although the multilayer mirrors of the illumination and projection systems are optimized for reflection of the desired wavelength e.g. 13 nm, they are optically flat and have quite high reflectivities at IR, visible and UV wavelengths. It is therefore desirable to select from the source a relatively narrow band of frequencies for the projection beam. Even where the source has a relatively narrow emission line, it is desirable to reject radiation out of that line, especially at longer wavelengths. It has been proposed to use a thin membrane as a filter to perform this function. However, such a film is very delicate and becomes very hot, 200–300° C. or more, leading to high thermal stresses and cracking, sublimation and oxidation in the high power levels necessary in a lithographic projection apparatus. A membrane filter also generally absorbs at least 50% of the desired radiation.
EP 1197803 describes a lithographic projection apparatus wherein a grating spectral filter is used in the radiation system of the lithographic projection apparatus. This grating spectral filter is designed for passing radiation of desired wavelengths to form a projection beam and for deflecting radiation of undesired wavelengths. The grating spectral filter is substantially formed of a material having a complex refractive index close to unity at the desired wavelengths and includes silicon protrusions (this structure is ‘invisible’ for the EUV radiation). The protrusions have a laminar sawtooth profile or a laminar square wave profile (FIGS. 3 and 4 of EP 1197803, respectively).
In this arrangement, however, the Si protrusions of spectral filters of EP 1107803 may easily be damaged, which may have a detrimental effect on the optical filter properties. Furthermore, the infra red (IR) radiations may not efficiently be blocked with the optical element of EP 1197803.